ARM: dts: exynos: Correct S3C RTC bindings in Tiny4412

The S3C RTC requires 32768 Hz clock as input which is provided by PMIC.
However the PMIC is not described in DTS at all so at least add a
workaround to model its clock with fixed-clock.  This fixes S3C RTC and
silences dtbs_check warnings like:

  arch/arm/boot/dts/exynos4412-tiny4412.dt.yaml: rtc@10070000: clocks: [[7, 346]] is too short
  arch/arm/boot/dts/exynos4412-tiny4412.dt.yaml: rtc@10070000: clock-names: ['rtc'] is too short

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20200830135200.24304-32-krzk@kernel.org